This invention relates generally to control of air flow rates of blowers and fans, which will be referred to simply as "fans" hereinafter.
Among the known types of blower or fan equipment, there are one in which a great difference exists between the equipment planned capacity and the actual load and one in which a great fluctuation occurs between light load and heavy load. In such cases, it has heretofore been the common practice to bring about suitable conformity with the load by adjusting the flow rate by means such as a damper or a valve in the fan equipment.
In fan equipment of this character, however, the control loss is extremely great. For this reason, by variable-speed operation of the fan driving motor, operation under satisfactory control can be accomplished without the use of a damper or valve or, according to necessity, in conjunction with the use of a damper or valve, and a great saving in energy can be realized.
Here, the control systems enumerated below are representative examples of variable-speed control devices suitable for achieving such variable-speed operation. (1) Control by pole change motor
Control system adapted to a plant operating at only two or more different speeds.
(2) Coupling control
Control system in which a coupling (eddy-current coupling or fluid coupling) is connected to the output shaft of a motor running at constant speed, and the transmission of the shaft horsepower is controlled.
(3) Primary voltage control
Control system which varies the primary voltage of the motor and controls the generated torque of the motor thereby to obtain variable speed.
(4) Secondary voltage control
Control system which uses a wound-rotor type motor and controls its secondary voltage. Secondary resistance control and Scherbius control are possible.
(5) Armature voltage control
Control system which makes use of a commutatorless motor (thyrister motor) in the case of alternating-current adjustable speed drive and a direct-current motor in the case of direct-current adjustable speed drive and controls the voltage supplied to the armature.
(6) Primary variable frequency control
Control system which supplies power of variable frequency and variable voltage to control the motor speed.
Even among these control systems, the primary frequency control system, (6), is superior as an energy-saving system since it can reduce power consumption, being capable of carrying out variable-speed control without the addition of special modifications to an existing motor and ancillary machines.
For example, when the speed of a fan motor for a boiler (forced draft fan) is controlled by a primary frequency control equipment, the power consumption of the fan is greatly reduced in comparison with that of another control system such as, for example, the system of controlling the degree of opening of an inlet vane as indicated in a graph described hereinafter. Particularly in partial-load operation, the energy-saving effectiveness of the primary frequency system is remarkably exhibited.
However, a primary frequency control equipment is accompanied by the risk of the fan tripping because of an occurrence such as disturbance of the power supply (instantaneous drop in the voltage or the like), overloading of the control device, or trouble in the device, which in turn results in the stopping of the boiler. More specifically, as described more fully hereinafter in conjunction with an accompanying time chart, when an abnormal signal from the primary frequency control equipment is received, confirmation as to whether there is residual voltage of the motor is carried out over a confirmation time of several seconds, and thereafter the fan motor is switched over to a commercial power supply. As a result, the fan motor rapidly returns to its rated speed, and, since the inlet vane is in its fully open position of this time, a large quantity of air is sent into the boiler, the combustion of which becomes unstable, whereby the boiler must unavoidably and disadvantageously be stopped.